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Hydrogenated diamondlike carbon (DLC:H) thin films exhibit many interesting properties that can be tailored by controlling the composition and energy of the vapor fluxes used for their synthesis. This control can be facilitated by high electron density and/or high electron temperature plasmas that allow one to effectively tune the gas and surface chemistry during film growth, as well as the degree of ionization of the film forming species. The authors have recently demonstrated by adding Ne in an Ar-C high power impulse magnetron sputtering (HiPIMS) discharge that electron temperatures can be effectively increased to substantially ionize C species [Aijaz et al., Diamond Relat. Mater. 23, 1 (2012)]. The authors also developed an Ar-C2H2 HiPIMS process in which the high electron densities provided by the HiPIMS operation mode enhance gas phase dissociation reactions enabling control of the plasma and growth chemistry [Aijaz et al., Diamond Relat. Mater. 44, 117 (2014)]. Seeking to further enhance electron temperature and thereby promote electron impact induced interactions, control plasma chemical reaction pathways, and tune the resulting film properties, in this work, the authors synthesize DLC: H thin films by admixing Ne in a HiPIMS based Ar/C2H2 discharge. The authors investigate the plasma properties and discharge characteristics by measuring electron energy distributions as well as by studying discharge current characteristics showing an electron temperature enhancement in C2H2 based discharges and the role of ionic contribution to the film growth. These discharge conditions allow for the growth of thick (amp;gt;1 mu m) DLC: H thin films exhibiting low compressive stresses (similar to 0.5 GPa), high hardness (similar to 25 GPa), low H content (similar to 11%), and density in the order of 2.2 g/cm(3). The authors also show that film densification and change of mechanical properties are related to H removal by ion bombardment rather than subplantation. (C) 2016 American Vacuum Society.

We have developed a kinetic model for residual stress generation in thin films grown from energetic vapor fluxes, encountered, e.g., during sputter deposition. The new analytical model considers sub-surface point defects created by atomic peening, along with processes treated in already existing stress models for non-energetic deposition, i.e., thermally activated diffusion processes at the surface and the grain boundary. According to the new model, ballistically induced subsurface defects can get incorporated as excess atoms at the grain boundary, remain trapped in the bulk, or annihilate at the free surface, resulting in a complex dependence of the steady-state stress on the grain size, the growth rate, as well as the energetics of the incoming particle flux. We compare calculations from the model with in situ stress measurements performed on a series of Mo films sputter-deposited at different conditions and having different grain sizes. The model is able to reproduce the observed increase of compressive stress with increasing growth rate, behavior that is the opposite of what is typically seen under non-energetic growth conditions. On a grander scale, this study is a step towards obtaining a comprehensive understanding of stress generation and evolution in vapor deposited polycrystalline thin films. Published by AIP Publishing.

Thin film growth from the vapor phase has for a long time intrigued researchers endeavouring to unravel and understand atomistic surface processes that govern film formation. Their motivation has not been purely scientific, but also driven by numerous applications where this understanding is paramount to knowledge-based design of novel film materials with tailored properties.

Within the above framework, this thesis investigates growth of metal films on weakly bonding substrates, a combination of great relevance for applications concerning e.g., catalysis, graphene metallization and architectural glazing. When metal vapor condenses on weakly bonding substrates three dimensional islands nucleate, grow and coalesce prior to forming a continuous film. The combined effect of these initial growth stages on film formation and morphology evolution is studied using pulsed vapor fluxes for the model system Ag/SiO2. It is shown that the competition between island growth and coalescence completion determines structure evolution. The effect of the initial growth stages on film formation is also examined for the tilted columnar microstructure obtained when vapor arrives at an angle that deviates from the substrate surface normal. This is done using two metals with distinctly different nucleation behaviour, and the findings suggest that the column tilt angle is set by nucleation conditions in conjunction with shadowing of the vapor flux by adjacent islands. Vapor arriving at an angle can in addition result in films that exhibit preferred crystallographic orientations, both out-of-plane and in-plane. Their emergence is commonly described by an evolutionary growth model, which for some materials predict a double in-plane alignment that has not been observed experimentally. Here, an experiment is designed to replicate the model’s growth conditions, confirming the existence of double in-plane alignment.

New and added film functionalities can further be unlocked by alloying. Properties are then largely set by chemistry and atomic arrangement, where the latter can be affected by thermodynamics, kinetics and vapor flux modulation. Their combined effect on atomic arrangement is here unravelled by presenting a research methodology that encompasses high resolution vapor flux modulation, nanoscale structure v vi probes and growth simulations. The methodology is deployed to study the immiscible Ag-Cu and miscible Ag-Au model systems, for which it is shown that capping of Cu by Ag atoms via near surface diffusion processes and rough morphology of the Ag-Au growth front are the decisive structure forming processes in each respective system.

The results generated in this thesis are of relevance for tuning structure of metal films grown on weakly bonding substrates. They also indicate that improved growth models are required to accurately describe structure evolution and emergence of a preferred in-plane orientation in films where vapor arrives at an angle that deviates from the substrate surface normal. In addition, this thesis presents a methodology that can be used to identify and understand structure forming processes in multicomponent films, which may enable tailoring of atomic arrangement and related properties in technologically relevant material systems.

Abstract [en]

Pulsed, ionized vapour fluxes, generated from high power impulse magnetron sputtering (HiPIMS) discharges, are employed to study the effects of time-domain and energetic bombardment on the nucleation and coalescence characteristics during Volmer–Weber growth of metal (Ag) films on amorphous (SiO2) substrates. In situ monitoring of the film growth, by means of wafer curvature measurements and spectroscopic ellipsometry, is used to determine the film thickness where a continuous film is formed. This thickness decreases from ~210 to ~140 Å when increasing the pulsing frequency for a constant amount of material deposited per pulse or when increasing the amount of material deposited per pulse and the energy of the film forming species for a constant pulsing frequency. Estimations of adatom lifetimes and the coalescence times show that there are conditions at which these times are within the range of the modulation of the vapour flux. Thus, nucleation and coalescence processes can be manipulated by changing the temporal profile of the vapour flux. We suggest that other than for elucidating the atomistic mechanisms that control pulsed growth processes, the interplay between the time scales for diffusion, coalescence and vapour flux pulsing can be used as a tool to determine characteristic surface diffusion and island coalescence parameters.

Note

The previous status of the article was Manuscript and the working title was Time-domain and energetic bombardment effects on the nucleation and post-nucleation characteristics during none-quilibrium film synthesis.

Abstract [en]

The initial formation stages (i.e., island nucleation, island growth, and island coalescence) set characteristic length scales during growth of thin films from the vapour phase. They are, thus, decisive for morphological and microstructural features of films and nanostructures. Each of the initial formation stages has previously been well-investigated separately for the case of Volmer-Weber growth, but knowledge on how and to what extent each stage individually and all together affect the microstructural evolution is still lacking. Here we address this question using growth of Ag on SiO2 from pulsed vapour fluxes as a case study. By combining in situ growth monitoring, ex situ imaging and growth simulations we systematically study the growth evolution all the way from nucleation to formation of a continuous film and establish the effect of the vapour flux time domain on the scaling behaviour of characteristic growth transitions (elongation transition, percolation and continuous film formation). Our data reveal a pulsing frequency dependence for the characteristic film growth transitions, where the nominal transition thickness decreases with increasing pulsing frequency up to a certain value after which a steady-state behaviour is observed. The scaling behaviour is shown to result from differences in island sizes and densities, as dictated by the initial film formation stages. These differences are determined solely by the interplay between the characteristics of the vapour flux and time required for island coalescence to be completed. In particular, our data provide evidence that the steady-state scaling regime of the characteristic growth transitions is caused by island growth that hinders coalescence from being completed, leading to a coalescence-free growth regime.

Abstract [en]

The tilt of the columnar microstructure has been studied for Cu and Cr thin films grown off-normally using highly ionized vapor fluxes, generated by the deposition technique high power impulse magnetron sputtering. It is found that the relatively large column tilt (with respect to the substrate normal) observed for Cu films decreases as the ionization degree of the deposition flux increases. On the contrary, Cr columns are found to grow relatively close to the substrate normal and the column tilt is independent from the ionization degree of the vapor flux when films are deposited at room temperature. The Cr column tilt is only found to be influenced by the ionized fluxes when films are grown at elevated temperatures, suggesting that film morphology during the film nucleation stage is also important in affecting column tilt. A phenomenological model that accounts for the effect of atomic shadowing at different nucleation conditions is suggested to explain the results.

Abstract [en]

The scientific interest and technological relevance of biaxially textured polycrystalline thin films stem from their microstructure that resembles that of single crystals. To explain the origin and predict the type of biaxial texture in off-normally deposited films, Mahieu et al. have developed an analytical model [S. Mahieu et al., Thin Solid Films 515, 1229 (2006)]. For certain materials, this model predicts the occurrence of a double in-plane alignment, however, experimentally only a single in-plane alignment has been observed and the reason for this discrepancy is still unknown. The model calculates the resulting in-plane alignment by considering the growth of faceted grains with an out-of-plane orientation that corresponds to the predominant film out-of-plane texture. This approach overlooks the fact that in vapor condensation experiments where growth kinetics is limited and only surface diffusion is active, out-of-plane orientation selection is random during grain nucleation and happens only upon grain impingement. Here, we compile and implement an experiment that is consistent with the key assumptions set forth by the in-plane orientation selection model by Mahieu et al.; a Cr film is grown off-normally on a fiber textured Ti epilayer to pre-determine the out-of-plane orientation and only allow for competitive growth with respect to the in-plane alignment. Our results show unambiguously a biaxially textured Cr (110) film that possesses a double in-plane alignment, in agreement with predictions of the in-plane selection model. Thus, a long standing discrepancy in the literature is resolved, paving the way towards more accurate theoretical descriptions and hence knowledge-based control of microstructure evolution in biaxially textured thin films.

Abstract [en]

Physical attributes of multicomponent materials of a given chemical composition are determined by atomic arrangement at property-relevant length scales. A potential route to access a vast array of atomic configurations for material property tuning is by synthesis of multicomponent thin films using vapor fluxes with their deposition pattern modulated in the sub-monolayer regime. However, the applicability of this route for creating new functional materials is impeded by the fact that a fundamental understanding of the combined effect of sub-monolayer flux modulation, kinetics and thermodynamics on atomic arrangement is not available in the literature. Here we present a research strategy and verify its viability for addressing the aforementioned gap in knowledge. This strategy encompasses thin film synthesis using a route that generates multi-atomic fluxes with sub-monolayer resolution and precision over a wide range of experimental conditions, deterministic growth simulations and nanoscale micro structural probes. Investigations are focused on structure formation within the archetype immiscible Ag-Cu binary system, revealing that atomic arrangement at different length scales is governed by the arrival pattern of the film forming species, in conjunction with diffusion of near-surface Ag atoms to encapsulate 3D Cu islands growing on 2D Ag layers. The knowledge generated and the methodology presented herein provides the scientific foundation for tailoring atomic arrangement and physical properties in a wide range of miscible and immiscible multinary systems. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Abstract [en]

We employ sub-monolayer, pulsed Ag and Au vapor fluxes, along with deterministic growth simulations, and nanoscale probes to study structure formation in miscible Ag-Au films synthesized under far-from-equilibrium conditions. Our results show that nanoscale atomic arrangement is primarily determined by roughness build up at the film growth front, whereby larger roughness leads to increased intermixing between Ag and Au. These findings suggest a different structure formation pathway as compared to the immiscible Ag-Cu system for which the present study, in combination with previously published data, reveals that no significant roughness is developed, and the local atomic structure is predominantly determined by the tendency of Ag and Cu to phase-separate.

Identifiers

Note

Funding agencies: Linkoping University via the "LiU Research Fellows Program"; Linkoping University via the "LiU Career Contract" [Dnr-LiU-2015-01510]; Swedish Research Council [VR-2011-5312, VR-2015-04630]

Physical attributes of multicomponent materials of a given chemical composition are determined by atomic arrangement at property-relevant length scales. A potential route to access a vast array of atomic configurations for material property tuning is by synthesis of multicomponent thin films using vapor fluxes with their deposition pattern modulated in the sub-monolayer regime. However, the applicability of this route for creating new functional materials is impeded by the fact that a fundamental understanding of the combined effect of sub-monolayer flux modulation, kinetics and thermodynamics on atomic arrangement is not available in the literature. Here we present a research strategy and verify its viability for addressing the aforementioned gap in knowledge. This strategy encompasses thin film synthesis using a route that generates multi-atomic fluxes with sub-monolayer resolution and precision over a wide range of experimental conditions, deterministic growth simulations and nanoscale micro structural probes. Investigations are focused on structure formation within the archetype immiscible Ag-Cu binary system, revealing that atomic arrangement at different length scales is governed by the arrival pattern of the film forming species, in conjunction with diffusion of near-surface Ag atoms to encapsulate 3D Cu islands growing on 2D Ag layers. The knowledge generated and the methodology presented herein provides the scientific foundation for tailoring atomic arrangement and physical properties in a wide range of miscible and immiscible multinary systems. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We employ sub-monolayer, pulsed Ag and Au vapor fluxes, along with deterministic growth simulations, and nanoscale probes to study structure formation in miscible Ag-Au films synthesized under far-from-equilibrium conditions. Our results show that nanoscale atomic arrangement is primarily determined by roughness build up at the film growth front, whereby larger roughness leads to increased intermixing between Ag and Au. These findings suggest a different structure formation pathway as compared to the immiscible Ag-Cu system for which the present study, in combination with previously published data, reveals that no significant roughness is developed, and the local atomic structure is predominantly determined by the tendency of Ag and Cu to phase-separate.

The scientific interest and technological relevance of biaxially textured polycrystalline thin films stem from their microstructure that resembles that of single crystals. To explain the origin and predict the type of biaxial texture in off-normally deposited films, Mahieu et al. have developed an analytical model [S. Mahieu et al., Thin Solid Films 515, 1229 (2006)]. For certain materials, this model predicts the occurrence of a double in-plane alignment, however, experimentally only a single in-plane alignment has been observed and the reason for this discrepancy is still unknown. The model calculates the resulting in-plane alignment by considering the growth of faceted grains with an out-of-plane orientation that corresponds to the predominant film out-of-plane texture. This approach overlooks the fact that in vapor condensation experiments where growth kinetics is limited and only surface diffusion is active, out-of-plane orientation selection is random during grain nucleation and happens only upon grain impingement. Here, we compile and implement an experiment that is consistent with the key assumptions set forth by the in-plane orientation selection model by Mahieu et al.; a Cr film is grown off-normally on a fiber textured Ti epilayer to pre-determine the out-of-plane orientation and only allow for competitive growth with respect to the in-plane alignment. Our results show unambiguously a biaxially textured Cr (110) film that possesses a double in-plane alignment, in agreement with predictions of the in-plane selection model. Thus, a long standing discrepancy in the literature is resolved, paving the way towards more accurate theoretical descriptions and hence knowledge-based control of microstructure evolution in biaxially textured thin films.

Thin films are nanoscale layers of material, with exotic properties useful in diverse areas, ranging from biomedicine to nanoelectronics and surface protection. Film properties are not only determined by their chemical composition, but also by their microstructure and roughness, features that depend crucially on the growth process due to the inherent out-of equilibrium nature of the film deposition techniques. This fact suggest that it is possible to control film growth, and in turn film properties, in a knowledge-based manner by tuning the deposition conditions. This requires a good understanding of the elementary film-forming processes, and the way by which they are affected by atomic-scale kinetics. The kinetic Monte Carlo (kMC) method is a simulation tool that can model film evolution over extended time scales, of the order of microseconds, and beyond, and thus constitutes a powerful complement to experimental research aiming to obtain an universal understanding of thin film formation and morphological evolution.

In this work, kMC simulations, coupled with analytical modelling, are used to investigate the early stages of formation of metal films and nanostructures supported on weakly-interacting substrates. This starts with the formation and growth of faceted 3D islands, that relies first on facile adatom ascent at single-layer island steps and subsequently on facile adatom upward diffusion from the base to the top of the island across its facets. Interlayer mass transport is limited by the rate at which adatoms cross from the sidewall facets to the island top, a process that determines the final height of the islands and leads non-trivial growth dynamics, as increasing temperatures favour 3D growth as a result of the upward transport. These findings explain the high roughness observed experimentally in metallic films grown on weakly-interacting substrates at high temperatures.

The second part of the study focus on the next logical step of film formation, when 3D islands come into contact and fuse into a single one, or coalesce. The research reveals that the faceted island structure governs the macroscopic process of coalescence as well as its dynamics, and that morphological changes depend on 2D nucleation on the II facets. In addition, deposition during coalescence is found to accelerate the process and modify its dynamics, by contributing to the nucleation of new facets.

This study provides useful knowledge concerning metal growth on weakly-interacting substrates, and, in particular, identifies the key atomistic processes controlling the early stages of formation of thin films, which can be used to tailor deposition conditions in order to achieve films with unique properties and applications.

Abstract [en]

Vapor condensation on weakly interacting substrates leads to the formation of three-dimensional (3D) nanoscale islands (i.e., nanostructures). While it is widely accepted that this process is driven by minimization of the total film/substrate surface and interface energy, current film-growth theory cannot fully explain the atomic-scale mechanisms and pathways by which 3D island formation and morphological evolution occurs. Here, we use kinetic Monte Carlo simulations to describe the dynamic evolution of single-island shapes during deposition of Ag on weakly interacting substrates. The results show that 3D island shapes evolve in a self-similar manner, exhibiting a constant height-to-radius aspect ratio, which is a function of the growth temperature. Furthermore, our results reveal the following chain of atomic-scale events that lead to compact 3D island shapes: 3D nuclei are first formed due to facile adatom ascent at single-layer island steps, followed by the development of sidewall facets bounding the islands, which in turn facilitates upward diffusion from the base to the top of the islands. The limiting atomic process which determines the island height, for a given number of deposited atoms, is the temperature-dependent rate at which adatoms cross from sidewall facets to the island top. The overall findings of this study provide insights into the directed growth of metal nanostructures with controlled shapes on weakly interacting substrates, including two-dimensional crystals, for use in catalytic and nanoelectronic applications.

Thin films are structures consisting of one or several nanoscale atomic layers of material that are used to either functionalize a surface or constitute components in more complex devices. Many properties of a film are closely related to its microstructure, which allows films to be tailored to meet specific technological requirements. Atom-by-atom film growth from the vapor phase involves a multitude of atomic processes that may not be easily studied experimentally in real-time because they occur in small length- (≤ Å) and timescales (≤ ns). Therefore, different types of computer simulation methods have been developed in order to test theoretical models of thin film growth and unravel what experiments cannot show. In order to compare simulated and experimental results, the simulations must be able to model events on experimental time-scales, i.e. on the order of microseconds to seconds. This is achievable with the kinetic Monte Carlo (kMC) method.

In this work, the initial growth stages of metal deposition on weakly-interacting substrates is studied using both kMC simulations as well as experiments whereby growth was monitored using in situ probes. Such film/substrate material combinations are widely encountered in technological applications including low-emissivity window coatings to parts of microelectronics components. In the first part of this work, a kMC algorithm was developed to model the growth processes of island nucleation, growth and coalescence when these are functions of deposition parameters such as the vapor deposition rate and substrate temperature. The dynamic interplay between these growth processes was studied in terms of the scaling behavior of the film thickness at the elongation transition, for both continuous and pulsed deposition fluxes, and revealed in both cases two distinct growth regimes in which coalescence is either active or frozen out during deposition. These growth regimes were subsequently confirmed in growth experiments of Ag on SiO2, again for both pulsed and continuous deposition, by measuring the percolation thickness as well as the continuous film formation thickness. However, quantitative agreement with regards to scaling exponents in the two growth regimes was not found between simulations and experiments, and this prompted the development of a method to determine the elongation transition thickness experimentally. Using this method, the elongation transition of Ag on SiO2 was measured, with scaling exponents found in much better agreement with the simulation results. Further, these measurement data also allowed the calculation of surface properties such as the terrace diffusion barrier of Ag on SiO2 and the average island coalescence rate.

In the second part of this thesis, pioneering work is done to develop a fully atomistic, on-lattice model which describes the growth of Ag on weakly-interacting substrates. Simulations performed using this model revealed several key atomic-scale processes occurring at the film/substrate interface and on islands which govern island shape evolution, thereby contributing to a better understanding of how 3D island growth occurs at the atomic scale for a wide class of materials. The latter provides insights into the directed growth of metal nanostructures with controlled shapes on weakly-interacting substrates, including twodimensional crystals for use in catalytic and nano-electronic applications.

Abstract [en]

The initial formation stages (i.e., island nucleation, island growth, and island coalescence) set characteristic length scales during growth of thin films from the vapour phase. They are, thus, decisive for morphological and microstructural features of films and nanostructures. Each of the initial formation stages has previously been well-investigated separately for the case of Volmer-Weber growth, but knowledge on how and to what extent each stage individually and all together affect the microstructural evolution is still lacking. Here we address this question using growth of Ag on SiO2 from pulsed vapour fluxes as a case study. By combining in situ growth monitoring, ex situ imaging and growth simulations we systematically study the growth evolution all the way from nucleation to formation of a continuous film and establish the effect of the vapour flux time domain on the scaling behaviour of characteristic growth transitions (elongation transition, percolation and continuous film formation). Our data reveal a pulsing frequency dependence for the characteristic film growth transitions, where the nominal transition thickness decreases with increasing pulsing frequency up to a certain value after which a steady-state behaviour is observed. The scaling behaviour is shown to result from differences in island sizes and densities, as dictated by the initial film formation stages. These differences are determined solely by the interplay between the characteristics of the vapour flux and time required for island coalescence to be completed. In particular, our data provide evidence that the steady-state scaling regime of the characteristic growth transitions is caused by island growth that hinders coalescence from being completed, leading to a coalescence-free growth regime.

Abstract [en]

The morphology of thin metal films and nanostructures synthesized from the vapor phase on insulating substrates is strongly influenced by the coalescence of islands. Here, we derive analytically the quantitative criterion for coalescence suppression by combining atomistic nucleation theory and a classical model of coalescence. Growth simulations show that using this criterion, a coalescence-free growth regime can be reached in which morphological evolution is solely determined by island nucleation, growth, and impingement. Experimental validation for the ability to control the rate of coalescence using this criterion and navigate between different growth regimes is provided by in situ monitoring of Ag deposition on SiO2. Our findings pave the way for creating thin films and nanostructures that exhibit a wide range of morphologies and physical attributes in a knowledge-based manner.

Abstract [en]

The morphology and physical properties of thin films deposited by vapor condensation on solid surfaces are predominantly set by the processes of island nucleation, growth, and coalescence. When deposition is performed using pulsed vapor fluxes, three distinct nucleation regimes are known to exist depending on the temporal profile of the flux. These regimes can be accessed by tuning deposition conditions; however, their effect on film microstructure becomes marginal when coalescence sets in and erases morphological features obtained during nucleation. By preventing coalescence from being completed, these nucleation regimes can be used to control microstructure evolution and thus access a larger palette of film morphological features. Recently, we derived the quantitative criterion to stop coalescence during continuous metal vapor flux deposition on insulating surfaceswhich typically yields 3-dimensional growthby describing analytically the competition between island growth by atomic incorporation and the coalescence rate of islands [Lu et al., Appl. Phys. Lett. 105, 163107 (2014)]. Here, we develop the analytical framework for entering a coalescence-free growth regime for metal vapor deposition on insulating substrates using pulsed vapor fluxes, showing that there exist three distinct criteria for suppressing coalescence that correspond to the three nucleation regimes of pulsed vapor flux deposition. The theoretical framework developed herein is substantiated by kinetic Monte Carlo growth simulations. Our findings highlight the possibility of using atomistic nucleation theory for pulsed vapor deposition to control morphology of thin films beyond the point of island density saturation. (C) 2015 AIP Publishing LLC.

Abstract [en]

The elongation transition thickness (hElong) is a central concept in the theoretical description of thin-film growth dynamics on weakly interacting substrates via scaling relations of hElong with respect to rates of key atomistic film-forming processes. To date, these scaling laws have only been confirmed quantitatively by simulations, while experimental proof has been left ambiguous as it has not been possible to measure hElong. Here, we present a method for determining experimentally hElong for Ag films growing on amorphous SiO2: an archetypical weakly interacting film/substrate system. Our results confirm the theoretically predicted hElong scaling behavior, which then allow us to calculate the rates of adatom diffusion and island coalescence completion, in good agreement with the literature. The methodology presented herein casts the foundation for studying growth dynamics and cataloging atomistic-process rates for a wide range of weakly interacting film/substrate systems. This may provide insights into directed growth of metal films with a well-controlled morphology and interfacial structure on 2D crystals-including graphene and MoS2-for catalytic and nanoelectronic applications. Published by AIP Publishing.

Place, publisher, year, edition, pages

American Institute of Physics (AIP), 2017

National Category

Condensed Matter Physics

Identifiers

Note

Funding Agencies|Linkoping University (LiU) [Dnr-LiU-2015-01510]; Swedish research council [VR-2011-5312, VR-2015-04630]; Swedish National Infrastructure for Computing (SNIC) at the National Supercomputer Centre (NSC)

Vapor condensation on weakly interacting substrates leads to the formation of three-dimensional (3D) nanoscale islands (i.e., nanostructures). While it is widely accepted that this process is driven by minimization of the total film/substrate surface and interface energy, current film-growth theory cannot fully explain the atomic-scale mechanisms and pathways by which 3D island formation and morphological evolution occurs. Here, we use kinetic Monte Carlo simulations to describe the dynamic evolution of single-island shapes during deposition of Ag on weakly interacting substrates. The results show that 3D island shapes evolve in a self-similar manner, exhibiting a constant height-to-radius aspect ratio, which is a function of the growth temperature. Furthermore, our results reveal the following chain of atomic-scale events that lead to compact 3D island shapes: 3D nuclei are first formed due to facile adatom ascent at single-layer island steps, followed by the development of sidewall facets bounding the islands, which in turn facilitates upward diffusion from the base to the top of the islands. The limiting atomic process which determines the island height, for a given number of deposited atoms, is the temperature-dependent rate at which adatoms cross from sidewall facets to the island top. The overall findings of this study provide insights into the directed growth of metal nanostructures with controlled shapes on weakly interacting substrates, including two-dimensional crystals, for use in catalytic and nanoelectronic applications.

The morphology and physical properties of thin films deposited by vapor condensation on solid surfaces are predominantly set by the processes of island nucleation, growth, and coalescence. When deposition is performed using pulsed vapor fluxes, three distinct nucleation regimes are known to exist depending on the temporal profile of the flux. These regimes can be accessed by tuning deposition conditions; however, their effect on film microstructure becomes marginal when coalescence sets in and erases morphological features obtained during nucleation. By preventing coalescence from being completed, these nucleation regimes can be used to control microstructure evolution and thus access a larger palette of film morphological features. Recently, we derived the quantitative criterion to stop coalescence during continuous metal vapor flux deposition on insulating surfaceswhich typically yields 3-dimensional growthby describing analytically the competition between island growth by atomic incorporation and the coalescence rate of islands [Lu et al., Appl. Phys. Lett. 105, 163107 (2014)]. Here, we develop the analytical framework for entering a coalescence-free growth regime for metal vapor deposition on insulating substrates using pulsed vapor fluxes, showing that there exist three distinct criteria for suppressing coalescence that correspond to the three nucleation regimes of pulsed vapor flux deposition. The theoretical framework developed herein is substantiated by kinetic Monte Carlo growth simulations. Our findings highlight the possibility of using atomistic nucleation theory for pulsed vapor deposition to control morphology of thin films beyond the point of island density saturation. (C) 2015 AIP Publishing LLC.

The elongation transition thickness (hElong) is a central concept in the theoretical description of thin-film growth dynamics on weakly interacting substrates via scaling relations of hElong with respect to rates of key atomistic film-forming processes. To date, these scaling laws have only been confirmed quantitatively by simulations, while experimental proof has been left ambiguous as it has not been possible to measure hElong. Here, we present a method for determining experimentally hElong for Ag films growing on amorphous SiO2: an archetypical weakly interacting film/substrate system. Our results confirm the theoretically predicted hElong scaling behavior, which then allow us to calculate the rates of adatom diffusion and island coalescence completion, in good agreement with the literature. The methodology presented herein casts the foundation for studying growth dynamics and cataloging atomistic-process rates for a wide range of weakly interacting film/substrate systems. This may provide insights into directed growth of metal films with a well-controlled morphology and interfacial structure on 2D crystals-including graphene and MoS2-for catalytic and nanoelectronic applications. Published by AIP Publishing.

Intrinsic stresses in vapor deposited thin films have been a topic of considerable scientific and technological interest owing to their importance for functionality and performance of thin film devices. The origin of compressive stresses typically observed during deposition of polycrystalline metal films at conditions that result in high atomic mobility has been under debate in the literature in the course of the past decades. In this study, we contribute towards resolving this debate by investigating the grain size dependence of compressive stress magnitude in dense polycrystalline Mo films grown by magnetron sputtering. Although Mo is a refractory metal and hence exhibits an intrinsically low mobility, low energy ion bombardment is used during growth to enhance atomic mobility and densify the grain boundaries. Concurrently, the lateral grain size is controlled by using appropriate seed layers on which Mo films are grown epitaxially. The combination of in situ stress monitoring with ex situ microstructural characterization reveals a strong, seemingly linear, increase of the compressive stress magnitude on the inverse grain size and thus provides evidence that compressive stress is generated in the grain boundaries of the film. These results are consistent with models suggesting that compressive stresses in metallic films deposited at high homologous temperatures are generated by atom incorporation into and densification of grain boundaries. However, the underlying mechanisms for grain boundary densification might be different from those in the present study where atomic mobility is intrinsically low. (C) 2016 AIP Publishing LLC.

The scientific and technological interest for metal-dielectric nanocomposite thin films emanates from the excitation of localized surface plasmon resonances (LSPRs) on the metal component. The overall optical response of the nanocomposite is governed by the refractive index of the dielectric matrix and the properties of the metallic nanoparticles in terms of their bulk optical properties, size, and shape, and the inter-particle distance of separation. In order to tune the film morphology and optical properties, complex synthesis processes which include multiple steps-i. e., film deposition followed by post-deposition treatment by thermal or laser annealing-are commonly employed. In the present study, we demonstrate that the absorption resonances of Ag/AlOxNy nanocomposite films can be effectively tuned from green (similar to 2.4 eV) to violet (similar to 2.8 eV) using a single-step synthesis process that is based on modulating the arrival pattern of film forming species with sub-monolayer resolution, while keeping the amount of Ag in the films constant. Our data indicate that the optical response of the films is the result of LSPRs on isolated Ag nanoparticles that are seemingly shifted by dipolar interactions between neighboring particles. The synthesis strategy presented may be of relevance for enabling integration of plasmonic nanocomposite films on thermally sensitive substrates. Published by AIP Publishing.

The authors investigate sputtering of a Ti3SiC2 compound target at temperatures ranging from RT (no applied external heating) to 970 °C as well as the influence of the sputtering power at 850 °C for the deposition of Ti3SiC2 films on Al2O3(0001) substrates. Elemental composition obtained from time-of-flight energy elastic recoil detection analysis shows an excess of carbon in all films, which is explained by differences in the angular distribution between C, Si, and Ti, where C scatters the least during sputtering. The oxygen content is 2.6 at. % in the film deposited at RT and decreases with increasing deposition temperature, showing that higher temperatures favor high purity films. Chemical bonding analysis by x-ray photoelectron spectroscopy shows C–Ti and Si–C bonding in the Ti3SiC2 films and Si–Si bonding in the Ti3SiC2 compound target. X-ray diffraction reveals that the phases Ti3SiC2, Ti4SiC3, and Ti7Si2C5 can be deposited from a Ti3SiC2 compound target at substrate temperatures above 850 °C and with the growth of TiC and the Nowotny phase Ti5Si3Cx at lower temperatures. High-resolution scanning transmission electron microscopy shows epitaxial growth of Ti3SiC2, Ti4SiC3, and Ti7Si2C5 on TiC at 970 °C. Four-point probe resistivity measurements give values in the range ∼120 to ∼450 μΩ cm and with the lowest values obtained for films containing Ti3SiC2, Ti4SiC3, and Ti7Si2C5.

We report how the total carbon content and the amorphous carbon (a-C) phase fraction in transition metal carbide/a-C nanocomposite coatings can be obtained using optical methods, which are much more practical for industrial use than conventional X-ray photoelectron spectroscopy. A large set of carbon-containing nanocomposite coatings deposited using different magnetron sputtering techniques were analyzed by X-ray photoelectron spectroscopy, reflectance spectrophotometry, and spectroscopic ellipsometry. The chemical composition and the a-C phase fraction were determined by X-ray photoelectron spectroscopy for each coating and results are presented for the Ti-C, Cr-C, and Nb-C systems. The composition and the a-C phase fraction are correlated to optical reflectance in the visible range, by parametrization in L*a*b* color space, and by ellipsometry primary data. Results show that it is possible to rapidly estimate the composition and the a-C fraction using these optical methods. We propose that optical methods have promising use in the industry as a cost-efficient technique for characterization of carbide-based coatings. (C) 2017 Elsevier B.V. All rights reserved.

Metastable solid solutions are phases that are synthesized far from thermodynamic equilibrium and offer a versatile route to design materials with tailor-made functionalities. One of the most investigated classes of metastable solid solutions with widespread technological implications is vapor deposited ternary transition metal ceramic thin films (i.e., nitrides, carbides, and borides). The vapor-based synthesis of these ceramic phases involves complex and difficult to control chemical interactions of the vapor species with the growing film surface, which often makes the fundamental understanding of the composition-properties relations a challenging task. Hence, in the present study, we investigate the phase stability within an immiscible binary thin film system that offers a simpler synthesis chemistry, i.e., the Ag-Mo system. We employ magnetron co-sputtering to grow Ag1-xMox thin films over the entire composition range along with x-ray probes to investigate the films structure and bonding properties. Concurrently, we use density functional theory calculations to predict phase stability and determine the effect of chemical composition on the lattice volume and the electronic properties of Ag-Mo solid solutions. Our combined theoretical and experimental data show that Mo-rich films (x &gt;= similar to 0.54) form bcc Mo-Ag metastable solid solutions. Furthermore, for Ag-rich compositions (x &lt;= similar to 0.21), our data can be interpreted as Mo not being dissolved in the Ag fcc lattice. All in all, our data show an asymmetry with regards to the mutual solubility of Ag and Mo in the two crystal structures, i.e., Ag has a larger propensity for dissolving in the bcc-Mo lattice as compared to Mo in the fcc-Ag lattice. We explain these findings in light of isostructural short-range clustering that induces energy difference between the two (fcc and bcc) metastable phases. We also suggest that the phase stability can be explained by the larger atomic mobility of Ag atoms as compared to that of Mo. The mechanisms suggested herein may be of relevance for explaining phase stability data in a number of metastable alloys, such as ternary transition metal-aluminum-nitride systems. (C) 2016 AIP Publishing LLC.